[sdiy] Sallen-Key Bessel Low Pass - behaviour and RC values
music.maker at gte.net
Thu Feb 6 15:03:51 CET 2014
Justin Owen <juzowen at gmail.com> wrote:
>I've been asked to help set up an anti-alias (input) and
>reconstruction (output) filter for a micro-based project a friend
>is working on. I'll be using a Bessel type Sallen-Key Low Pass
>config for both - but I had some initial questions.
>In a general sense, my understanding is that the real world
>differences in behaviour/performance between Bessel and
>Butterworth in a SallenKey config are mainly down to Q and
>...and the differences in component selection is that for a
>Butterworth topology there is a 2:1 ratio between the values of
>C1 and C2 (or C2 and C ATCMB...) and for a Bessel C1 = C2.
>Is that right?
>Secondly, there seems to be two schools of thought on whether to
>set the filter cutoff frequency at Nyquist (which I believe is
>currently 96KHz) or around the highest frequency the DAC will
>output (in our case I believe this is about 16KHz).
I will assume this is an audio project. As such, I don't believe we need much
more bandwidth than that of human hearing. As such, a cut off of 96 kHz seems a
bit extreme. I would put cutoff somewhere between 16 kHz and 20 kHz.
>Any thoughts on this from those who've been round this block before?
>Lastly - I hope you guys don't me asking this on list but I'd be
>curious to hear from Tom W. and Olivier G. about their RC
>selections of 12K/10nF (plus some gain?) and 39K/100pF - and
>anyone else on there fave or suggested RC values.
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